In this specification, the terms circuit assembly and printed circuit board will be considered interchangeable. The term circuit assembly includes printed circuit boards as well as other types of circuit assemblies. A circuit assembly is a combination of electrical and electronic components and the electrical conductors that connect those components. The resulting combination is manufactured to form a physical or functional unit. In this discussion the term trace is used to describe these connecting electrical conductors, particularly on a printed circuit board. The term node includes the electrical connections comprising such a conductively connected path, including the connection pins of the components. Generally, the term node does not include circuitry inside of the components that is not conductively connected to the terminal connections of said components.
In this specification, the term integrated circuit is used to describe a class of components attached to a circuit assembly. This term is not used to limit the application of the disclosed invention to integrated circuits. The current state of the art is such that a major contribution of this invention is in testing components which are integrated circuits.
It is important that electronic components and printed circuit boards be tested after the components have been soldered to the printed circuit boards. Several different approaches have been developed for testing the components and printed circuit boards, including functional testing, in-circuit testing, and manufacturing defect analysis.
Functional testing uses a procedure of applying predetermined input signals and monitoring the output of a printed circuit board to determine if all of the components are present and operating properly on the circuit board. While functional testing provides a way of determining whether the printed circuit board is functioning properly, it provides little or no information regarding the functioning of individual components on the board. Complex programming techniques have been used to provide limited information as to the location of non-functioning components on the board by carefully selecting input data and analyzing the output results. Such systems are complex, often costly to implement, and normally provide only vague information as to the location of malfunctioning components.
Because of the limitations of functional testing, in-circuit testing techniques have been used to individually test the components on the printed circuit board to determine if these components are installed and working properly. This process uses a "bed of nails" fixture to access each individual component and test that component individually. In this manner, non-functioning components can be identified and replaced to prevent the entire circuit board from being scrapped. This process works well for simple components where the circuit inside the component is known and can be easily tested. If the component being tested is very complex, or if the circuit inside the component is unknown, in-circuit testing may not achieve satisfactory results.
Manufacturing defect analyzers are another class of testing devices that provide simpler tests and are less expensive to implement. These devices are designed to locate manufacturing faults, such as shorts on a printed circuit board, missing integrated circuits, bent component pins, etc. Although these devices do a reasonably good job of finding shorts and gross analog faults, they are marginally satisfactory for testing digital sections of the board.
One very important requirement that must be addressed for every printed circuit board is that all the pins of every component must be soldered to the circuit board. Functional testing may miss a particular pin if the functions performed by that particular pin are not thoroughly tested in the functional test. Testing for this type of fault is particularly difficult when the circuit inside the component is unknown, such as the case with application specific integrated circuits (ASICs). Because of the large number of ASICs and the complexity of these devices, it is often not feasible, for reasons of time or cost, to design an in-circuit test or a functional test to isolate a fault on this type of component.
Prior art in the area of connection verification by capacitive connection includes UK Patent GB 2143954A, by Michael Rignall. Rignall cites one of the main reasons for capacitive coupling in verifying networks of a circuit assembly or printed circuit board as being the lack of the need of an ohmic contact. It is recognized that ohmic contact probes are susceptible to contamination on the contacting surfaces. This contamination can cause improper test decisions to be made such that an open is diagnosed as being in the circuit under test when, in reality, the test contact is faulty and the circuit is not faulty. Rignall teaches that the connectivity of a conductive connection, isolated from other circuitry, can be verified by capacitive coupling to extreme ends of a conductive trace on the circuit assembly. Rignall teaches that such capacitive coupling allows the test to be made without a contacting connection to the trace.